1. Field of the Invention
This invention relates to analog-to-digital (A/D) converters. More particularly, this invention relates to such converters of the algorithmic type having a cascaded series of stages each producing a corresponding bit for an output digital word. Such a cascaded series of stages is sometimes referred to as a pipelined arrangement.
2. Description of the Prior Art
Algorithmic converters of various kinds have been known in the art for many years. A pipelined algorithmic A/D converter is described in the textbook "CMOS Analog Circuit Design" by Phillip E. Allen and Douglas R. Holberg (1987) at page 565 et seq. FIG. 1 of this application is reproduced from that description, and shows a cascaded one-bit/stage converter which carries out an algorithm wherein the input voltage to each stage is doubled and combined with plus or minus V.sub.REF, depending upon the sign of the input voltage to each stage as determined by a respective comparator. The signals for the pipelined stages are shifted between the stages under clock control, and the delay between shifts is diagrammatically represented by the blocks "Z.sup.-1 ".
Such algorithmic converters have a number of sources of error including comparator offset voltage and charge injection from transistor switches commonly used in carrying out the algorithm. The multiply-by-two function is conveniently performed by an amplifier using a pair of switchable capacitors. With such an amplifier circuit, gain errors will be caused by capacitor mismatch, resulting in differential nonlinearity (DNL) and integral nonlinearity (INL). The errors from comparator offset and charge injection can be substantially neutralized by employing a modified algorithm using two comparators per stage, to carry out a so-called 1.5 bit/stage algorithm. However, errors due to capacitor mismatch are not reduced by such an algorithm. The present invention is directed to minimizing errors from capacitor mismatch in such a converter by means of a self-calibration technique which functions entirely in the digital domain.